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<h1><a name="SECTION0033300000000000000000">
31.3 Python Abstract Syntax</a>
</h1>

<p>
The <tt class="module">compiler.ast</tt> module defines an abstract syntax for
Python.  In the abstract syntax tree, each node represents a syntactic
construct.  The root of the tree is <tt class="class">Module</tt> object.

<p>
The abstract syntax offers a higher level interface to parsed Python
source code.  The <a class="ulink" href="http://www.python.org/doc/current/lib/module-parser.html"
  ><tt class="module">parser</tt></a>
module and the compiler written in C for the Python interpreter use a
concrete syntax tree.  The concrete syntax is tied closely to the
grammar description used for the Python parser.  Instead of a single
node for a construct, there are often several levels of nested nodes
that are introduced by Python's precedence rules.

<p>
The abstract syntax tree is created by the
<tt class="module">compiler.transformer</tt> module.  The transformer relies on the
builtin Python parser to generate a concrete syntax tree.  It
generates an abstract syntax tree from the concrete tree.  

<p>
The <tt class="module">transformer</tt> module was created by Greg
Stein<a id='l2h-5497' xml:id='l2h-5497'></a> and Bill Tutt<a id='l2h-5498' xml:id='l2h-5498'></a> for an
experimental Python-to-C compiler.  The current version contains a
number of modifications and improvements, but the basic form of the
abstract syntax and of the transformer are due to Stein and Tutt.

<p>

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